The invention relates to pulverizers of the type that are used in pulverized coal fired steam generating systems. More particularly, the invention relates to pulverizers that will function well at both low load conditions and high load conditions.
Pulverized coal firing is favored over other methods of burning coal because pulverized coal burns like gas and, therefore, fires are easily lighted and controlled.
Pulverizers, also referred to as mills, are used to grind or comminute the fuel. The present invention has primary application to bowl pulverizers or mills. Although the present invention will be described with respect a bowl type ring roll pulverizer. Those skilled in the art will recognize that the invention may be used in other bowl type mills as well.
The force required to urge the cooperating elements in such pulverizers toward each other varies substantially with the load requirements. In other words at high load conditions (when there is a high coal flow rate), the force requirements are much greater than at low load conditions (when there is a low coal flow rate). Environmental concerns have been the basis for limiting emissions of nitrogen monoxide and nitrogen dioxide as byproducts of the combustion process used in steam generating systems. Systems to reduce such emissions have lowered the temperature of combustion tending to result in the undesirable side effect of increasing the carbon content in the ash. One countermeasure to reduce the carbon content of the ash is to provide finer particles of coal to the combustion process. In other words, in systems that meet the nitrogen monoxide and nitrogen dioxide emission requirement, if finer coal particles are burned in the combustion process the amount of carbon in the ash will less than if coarser coal particles are burned in the combustion process. Accordingly, there is an increasing demand for coal pulverizers that will produce the desired small particles.
Pulverizers operating at low load conditions to produce particles that are very fine tend to run roughly. This roughness is due to the relative thinness of the coal bed in the bowl in which the coal is being crushed. (It will be understood that the term "coal bed" refers to the layer of coal disposed intermediate the roll and the bowl in the pulverizer.) Conventional spring biased pulverizers have constructed with a design trade-off. On the one hand, the spring rate may be selected so as to be compatible with the low load condition. With this selection the spring rate will be inadequate at high load conditions. The inadequate spring rate means that the mill will produce less fineness at high load conditions and the mill will be unable to maintain the required throughput. The failure to maintain the required throughput will commonly result in spillage of the coal because the coal will overflow the bowl of the pulverizer.
On the other hand, if they spring rate is chosen to the appropriate for high feed rates the pulverizer will have too high a spring force at low flow rates. The effect of a spring rate that is too high is that too much power is required to drive the mill, there is excessive wear on the components resulting in increased maintenance and decreased component time to failure, excessive crushing of the coal resulting in excessive fineness and thus greater fan power requirements to blow the crushed coal into the combustion area.
Variable rate coil springs are known in relatively small sizes. Such variable rate springs may, for example, have axial sections of differing coil diameters or different spacing between turns or other characteristics that differ in separate axial sections. The springs required for pulverizers are much larger. The specific physical characteristics will vary with the application, however the spring rates may be in the order of 2000-40,000 pounds per inch of deflection, the spring may be manufactured from two or three inch bar stock, the diameter may be fourteen or sixteen inches, and the spring may be two feet long. Although these numbers are only representative, it will readily be seen and understood by those skilled in the art that variable rate springs having such very large sizes have not been produced because of the expense of tooling for their manufacture and because the reliability of such devices is not known.
It is an object of the invention to provide a pulverizer which will produce very fine coal particles at both low load conditions and high load conditions.
It is also an object of the invention to provide a mechanism that will produce a variable rate spring effect in an assembly that incorporates only constant rate springs.
It is still another object of the invention to produce apparatus which will be highly reliable.
Still another object of the invention is to produce an apparatus which is relatively inexpensive to manufacture.